The nation's biggest retailer will open its doors on Thanksgiving night, two hours earlier than in 2011.

Walmart sent out notice Thursday that it will open its doors at 8 p.m. on Thanksgiving night, and retail expert Dennis Garrett of Marquette University said other chains will be forced to match that.

"This is like a nuclear arms race, but in the retail sector, and Walmart's leading the way. And when Walmart leads the way, nobody else can take the risk of letting Walmart get too far ahead," Garrett said.

With some retailers counting on holiday shoppers for up to 50 percent of their annual sales, Black Friday may be more important than ever. But shoppers disagreed over how soon is too soon to start.

"I think it's a little bit too early. I want to wait until after Thanksgiving," a shopper said.

Garrett said the stores aren't the only ones competing on Black Friday.

"The consumers too don't want to let their fellow consumers get a leg up, so they're going to be the first ones out there. It's become sort of a blood sport, really," Garrett said.

For some, Thursday may have become the new Black Friday. Garrett said shoppers may have expand the shopping calendar by next year.

"Maybe somebody's going to say, ‘Let's have a Wednesday before Thanksgiving sale.’ I think that's the next move in the works, and could come as early as next year frankly," Garrett said.

Some websites are already listing what they said are the leaked Black Friday sale flyers from dozens of retailers.

Garrett said about one-third of his students shopped on Thanksgiving in 2011. One said he started as early as noon. But about three-fourths said they thought the whole thing was getting out of hand.